The present invention relates to a tapered thread configuration particularly adapted for use in the pipe joints of oil well casing or the like.
In the drilling of oil wells large diameter pipe is lowered into the ground as drilling progresses to restrain the surrounding earth from entering the hole and to prevent the well products from escaping into the surrounding strata. Relatively short lengths of these pipe are joined together by threaded couplings to create the long casing "string" needed to ultimately reach the bottom of the well. Since these "strings" of casing can be as much as five miles in length it is obvious that the threads, in addition to providing a fluid tight joint, must be exceptionally strong in order to support the weight of such a great length of casing. Not only must the threads be strong in tension in order to suspend the string from above but they must also be capable of withstanding a compressive loading of substantially equal force, as occurs when the leading end of the casing encounters an obstruction in the hole through which the casing must be caused to break through. At these times the weight of the string may be dropped against the obstruction to effect a breakthrough, thus loading the threads in compression.
In practice additional pipe lengths are added to the casing string by rigidly restraining the uppermost pipe immediately below the coupling assembled at its upper end; thereafter; lowering the pipe to be added into the coupling and rotating it to make up the threads. Since each pipe length may weigh up to 2 tons, lowering the heavy, unwieldy pipe into the coupling often results in the threads crossing or jamming rather than mating. This, of course, requires that the members be freed and another attempt be made to mate them. In addition to adding to the cost of the operation, the danger to the surrounding area from well "blow out" is increased since "blow out" protection devices cannot be applied to the apparatus until the casing is completely in place.
It has been proposed to employ a thread with rounded crests and roots and an intermediate flank in large diameter pipe and coupling members in order to increase the tensile strength properties of the joint. Such a thread form is described in U.S. Pat. No. 2,909,380, issued Oct. 20, 1959 to E. C. Haye, et al. This proposed thread form cannot, however, perform adequately in applications such as those contemplated for use of the present invention where a coupling that is also strong in compression is required. Also, due to the asymmetrical nature of the thread form, members incorporating the same will be more difficult to fabricate than those incorporating a thread form as described herein.
It is to an improved form of threaded joint connections particularly for apparatus of the above type, therefore, that the present invention is directed.